Aluminum alloy sheet for can body, and process for producing the same

ABSTRACT

An Al alloy sheet for a can body and a method for manufacturing the Al alloy sheet are provided. The Al sheet has a predetermined Al alloy composition, and is configured so that the solid solution Mn content thereof after hot rolling is 0.25 mass % or greater, the solid solution Fe content is 0.02 mass % or greater, and the solid solution Si content is 0.07 mass % or greater. The electrical conductivity thereof is 30.0-40.0% IACS, the tensile strength in the rolling direction of a cold-rolled sheet thereof is 280-320 MPa, the tensile strength in the rolling direction after heat treatment at 205° C. for 10 minutes is 270-310 MPa, and the difference between the tensile strength in the rolling direction and the yield stress in the rolling direction after heat treatment at 205° C. for 10 minutes is 50 MPa or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 15/992,662,filed May 30, 2018, which in turn is a Continuation of InternationalApplication No. PCT/JP2016/088086, filed Dec. 21, 2016, the entiretiesof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to an aluminum alloy sheet for acan body and a process for producing the same, and more particularly toan aluminum alloy sheet which can exhibit excellent properties inproduction of the can body, and a process for advantageously producingthe same.

Description of Related Art

Conventionally, an aluminum (Al) alloy sheet for a can body has beenproduced by subjecting an Al alloy ingot to homogenization, hot rollingand cold rolling, as known well. Then, the Al alloy sheet for the canbody is subjected to degreasing and washing, and oil-coating, asnecessary, and further subjected to cupping, DI (Drawing and Ironing)forming, trimming, cleaning, drying, coating, baking, necking, flangingand the like, so that a can body for beverages and the like is produced.

Meanwhile, the can body for beverages and the like is required to have apractically sufficient strength, while the body strength issignificantly deteriorated in the above-described baking step after thecoating step (hereinafter referred to as coating and baking process).Thus, various means to prevent the deterioration of the strength of thecan body, for example the following techniques, have been proposed.

JP2012-92431A1 (Patent Document 1) discloses an aluminum alloycold-rolled sheet for a bottle can, wherein an amount of dispersedparticles with a barycentric diameter of less than 1 μm represented asan α phase in a sheet structure is reduced, and a proportion of a βphase being an Al₆(Fe, Mn)-based intermetallic compound and the α phasebeing an Al—Fe—Mn—Si-based intermetallic compound is set to be not lessthan 0.50 in terms of Hβ/Hα, Hα and Hβ being a largest height of anX-ray diffraction peak of the α phase and the β phase respectively, sothat a hot-rolled sheet is given uniform recrystallization in the widthdirection, and variations in earing ratio of the sheet in the widthdirection can be reduced accordingly.

Furthermore, JP2011-202273A1 (Patent Document 2) discloses an aluminumalloy cold-rolled sheet for a bottle can, which has a specific alloycomposition, in which amounts of solid-solubilized or dissolved Fe andMn in a sheet structure is suitably controlled, to thereby increase anumber density of relatively large dispersed particles which function asnucleation sites for recrystallization in a hot-rolled sheet, whilepromoting the recrystallization in a central part of the rolled sheet inthe width direction, in particular in a middle part of the sheet in thethickness direction, and permitting uniform recrystallization of thehot-rolled sheet in the width direction, so that the variation of theearing ratio of the sheet in the width direction can be reduced.

Meanwhile, in recent years, it has become an important issue to recyclea reclaimed mass of used beverage cans (UBC: Used Beverage Can) inproducing can bodies for beverages, in view of protection of theenvironment. Furthermore, to reduce an amount of use of materials,endeavors have been made to reduce the thickness and weight of canbodies. However, the reclaimed mass of UBC often contains Si, Fe and thelike, so that an Al alloy ingot obtained by recycling the reclaimed massof UBC includes a high concentration of Si and Fe. Si forms anintermetallic compound with Mn and Fe at the time of heating of the Alalloy ingot, resulting in a decrease of the amount of solid-solubilizedMn. As a result, thermal softening resistance of an obtained Al alloysheet is reduced, thereby causing a problem that the strength of the canbody is significantly deteriorated in the coating and baking process. Inthe case where the initial strength of the Al alloy sheet is set to behigh in consideration of the reduction of the strength during thecoating and baking process, there arises a problem that a risk ofbreakage of the can body during the DI forming increases with a decreaseof the thickness and weight of a wall portion of the can body.Accordingly, in production of the can bodies for beverages by recyclingthe reclaimed mass of UBC, the amount of use of the reclaimed mass ofUBC must be limited and a new base metal mass must be added, to therebycontrol a content of Si in the alloy ingot.

It is noted that Patent Document 1 discloses a technique of suppressinggeneration of precipitated particles (α phase) having a size of lessthan 1 μm, and Patent Document 2 discloses a technique of controllingonly the amount of solid-solubilized Fe and Mn so as to promoterecrystallization during hot rolling, thereby controlling only theearing ratio. However, these techniques can neither achieve both of thedesired strength of the can body and the earing ratio at the same time,nor do they pay attention to the amounts of solid-solubilized Fe, Mn andSi, and to the fine particles (α phase) precipitated during coldrolling.

Patent Document 1: JP 2012-92431 A1

Patent Document 2: JP 2011-202273 A1

SUMMARY OF THE INVENTION

The present invention was made in view of the background art describedabove. Therefore, it is a technical problem of the invention to providean Al alloy sheet having excellent properties for use as a can body, anda process for producing the same. It is another problem of the inventionto provide an Al alloy sheet for use as a can body, and a process forproducing the same, wherein decrease of the strength of the can body, inparticular the strength of the can body after heat treatment, whichinfluences troubles such as deterioration of the body strength andbreakage of the body, is effectively suppressed by optimizing theamounts of solid-solubilized Fe, Mn and Si, and the fine particles (αphase) precipitated during cold rolling.

The above-described problems can be solved according to one mode of theinvention, which provides an aluminum alloy sheet for a can body whichis formed of a cold-rolled sheet obtained by cold rolling a hot-rolledsheet comprising an aluminum alloy consisting of 0.7-1.3% by mass of Mn,0.8-1.5% by mass of Mg, 0.25-0.6% by mass of Fe, 0.25-0.50% by mass ofSi, 0.10-0.30% by mass of Cu, not more than 0.25% by mass of Zn, notmore than 0.10% by mass of Ti and not more than 0.05% by mass of B, andthe balance being Al and inevitable impurities, wherein the hot-rolledsheet includes not less than 0.25% by mass of solid-solubilized Mn, notless than 0.02% by mass of solid-solubilized Fe and not less than 0.07%by mass of solid-solubilized Si, and has an electric conductivity of30.0-40.0% IACS, and wherein the cold-rolled sheet has a tensilestrength (TS) of 280-320 MPa in a rolling direction and a tensilestrength (ABTS) of 270-310 MPa in the rolling direction after heattreatment at 205° C. for 10 minutes, and a difference between thetensile strength (TS) in the rolling direction and a yield strength(ABYS) in the rolling direction after heat treatment at 205° C. for 10minutes is not larger than 50 MPa.

The above-described problems can also be solved according to anothermode of the invention, which provides an aluminum alloy hot-rolled sheetfor a can body, comprising an aluminum alloy consisting of 0.7-1.3% bymass of Mn, 0.8-1.5% by mass of Mg, 0.25-0.6% by mass of Fe, 0.25-0.50%by mass of Si, 0.10-0.30% by mass of Cu, not more than 0.25% by mass ofZn, not more than 0.10% by mass of Ti and not more than 0.05% by mass ofB, and the balance being Al and inevitable impurities, wherein thehot-rolled sheet includes not less than 0.25% by mass ofsolid-solubilized Mn, not less than 0.02% by mass of solid-solubilizedFe and not less than 0.07% by mass of solid-solubilized Si, and has anelectric conductivity of 30.0-40.0% IACS.

The above-described problems can also be solved according to a furthermode of the invention, which provides an aluminum alloy sheet for a canbody, comprising an aluminum alloy consisting of 0.7-1.3% by mass of Mn,0.8-1.5% by mass of Mg, 0.25-0.6% by mass of Fe, 0.25-0.50% by mass ofSi, 0.10-0.30% by mass of Cu, not more than 0.25% by mass of Zn, notmore than 0.10% by mass of Ti and not more than 0.05% by mass of B, andthe balance being Al and inevitable impurities, wherein the sheet has atensile strength (TS) of 280-320 MPa in a rolling direction and atensile strength (ABTS) of 270-310 MPa in the rolling direction afterheat treatment at 205° C. for 10 minutes, and a difference between thetensile strength (TS) in the rolling direction and a yield strength(ABYS) in the rolling direction after heat treatment at 205° C. for 10minutes is not larger than 50 MPa.

In a preferable form of the aluminum alloy sheet for a can bodyaccording to the above-described mode of the invention, the sheet has anelectric conductivity of 28.4% IACS-39.8% IACS.

The above-described aluminum alloy sheet for a can body according to theinvention is advantageously produced by a process comprising steps of:(a) providing an ingot of an aluminum alloy consisting of 0.7-1.3% bymass of Mn, 0.8-1.5% by mass of Mg, 0.25-0.6% by mass of Fe, 0.25-0.50%by mass of Si, 0.10-0.30% by mass of Cu, not more than 0.25% by mass ofZn, not more than 0.10% by mass of Ti and not more than 0.05% by mass ofB, and the balance being Al and inevitable impurities; (b) performinghot rolling on the ingot of an aluminum alloy so as to obtain ahot-rolled sheet including not less than 0.25% by mass ofsolid-solubilized Mn, not less than 0.02% by mass of solid-solubilizedFe and not less than 0.07% by mass of solid-solubilized Si, and havingan electric conductivity of 30.0-40.0% IACS; and (c) performing coldrolling on the hot-rolled sheet so as to form a cold-rolled sheetwherein a tensile strength (TS) in a rolling direction is 280-320 MPaand a tensile strength (ABTS) in the rolling direction after heattreatment at 205° C. for 10 minutes is 270-310 MPa, and wherein adifference between the tensile strength (TS) in the rolling directionand a yield strength (ABYS) in the rolling direction after heattreatment at 205° C. for 10 minutes is not larger than 50 MPa.

The above-described aluminum alloy sheet for a can body according to theinvention is also advantageously produced by a process comprising stepsof: surface-machining an ingot of an aluminum alloy consisting of0.7-1.3% by mass of Mn, 0.8-1.5% by mass of Mg, 0.25-0.6% by mass of Fe,0.25-0.50% by mass of Si, 0.10-0.30% by mass of Cu, not more than 0.25%by mass of Zn, not more than 0.10% by mass of Ti and not more than 0.05%by mass of B, and the balance being Al and inevitable impurities;heating the ingot to a homogenization temperature (T) within a range of550-620° C. at a heating rate of 30-120° C. per hour; performinghomogenization by keeping the ingot at the homogenization temperature(T) for a time not shorter than (145-0.24T) hours; performing rough hotrolling on the ingot immediately or after cooling the ingot to astarting temperature of hot rolling not lower than 500° C. at a coolingrate of 10-90° C. per hour after finishing the homogenization, such thata temperature of the ingot upon termination of the rough hot rolling iswithin a range of 430-550° C., so as to form a sheet having a thicknessof 20-40 mm; performing finish hot rolling on the sheet such that atemperature of the sheet upon termination of the finish hot rolling iswithin a range of 300-390° C., so that the sheet has a thickness of1.5-4.0 mm; and performing cold rolling on the sheet such that a totalworking ratio of the sheet is not less than 75% and an average rollingrate in a steady part of a final pass is within a range of 700-1600 mper minute, so that the sheet has a thickness of 0.2-1.0 mm.

In a preferable form of the process for producing the aluminum alloysheet for a can body according to the invention, the difference (S1-S2)between the electric conductivity (S1) of the sheet obtained by thefinish hot rolling and the electric conductivity (S2) of the sheetobtained by the cold rolling is controlled to be 0.2-1.6% IACS.

In another preferable form of the process for producing the aluminumalloy sheet for a can body according to the invention, an area ofparticles having a diameter of 0.1 μm-1 μm in the ingot of an aluminumalloy subjected to the homogenization is not less than 3.5% in terms ofa microphotograph taken by a scanning electron microscope.

According to the invention, there is also provided a can body forbeverages, which is formed of the above-described aluminum alloy sheet.

In a preferable form of the can body for beverages according to theinvention, the above-described aluminum alloy sheet for a can body issubjected to a predetermined coating and baking process.

As described above, the Al alloy sheet for the can body according to theinvention is formed of the Al alloy consisting of the specific alloycomposition, such that the amounts of solid-solubilized Fe, Mn and Si inthe hot-rolled sheet prepared from the Al alloy and precipitation offine particles (α phase) during cold rolling of the hot-rolled sheet areoptimized. For this reason, an excellent formability and a high thermalsoftening resistance of the sheet are realized simultaneously, andfurther an excellent strength of the can body is exhibited even afterthe heat treatment, so that the sheet is advantageously used as amaterial for the can body.

Furthermore, according to the process for producing the aluminum alloysheet for the can body according to the invention, the aluminum alloysheet for the can body having excellent properties such as compatibilityof formability and the strength of the can body after the heat treatmentcan be industrially advantageously produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the amount of variation of the electricconductivity when each of two aluminum alloy materials having differentalloy compositions was subjected to compression forming at a temperatureof 150° C.

DETAILED DESCRIPTION OF THE INVENTION

First of all, an aluminum alloy to provide an aluminum alloy sheet for acan body according to the invention includes 0.7-1.3% (by mass, the samefor the following) of Mn, 0.8-1.5% of Mg, 0.25-0.6% of Fe, 0.25-0.50% ofSi, 0.10-0.30% of Cu, not more than 0.25% of Zn, not more than 0.10% ofTi and not more than 0.05% of B, the balance being Al and inevitableimpurities. The reasons for limiting the amounts of the components areas follows.

[Mn: 0.7-1.3%]

Mn (manganese), which is an essential alloy element in the Al alloysheet according to the invention, improves the strength, and inparticular contributes to an improvement of the thermal softeningresistance in a solid-solubilized state. Mn forms an α-phase compound(Al—Mn—Fe—Si-base) with Fe and Si, which are also impurity elementsinevitably introduced in the production process. Particles of thisintermetallic compound have a quite high degree of hardness, and preventseizure between an Al alloy material and a mold in a forming process soas to improve surface properties of a formed container. In the casewhere a content of Mn is less than 0.7%, the effects of Mn are notsufficiently exhibited. On the other hand, in the case where the contentexceeds 1.3%, there arises a problem of an excessively high degree ofthe strength. The content of Mn is preferably within a range of0.8-1.2%.

[Mg: 0.8-1.5%]

Mg (magnesium) is solid-solubilized in Al so as to contribute to anincrease of the strength of the container. In the case where a contentof Mg is less than 0.8%, a sufficient strength suitable for an endproduct is difficult to be achieved. On the other hand, in the casewhere the content exceeds 1.5%, there arises a problem of deteriorationof formability because the strength of the can body becomes excessivelyhigh. The content of Mg is preferably within a range of 1.0-1.3%.

[Fe: 0.25-0.6%]

Fe (iron) forms, with Mn, an Al₆(Fe, Mn) phase compound and an α phase(Al—Mn—Fe—Si-based) compound, and also forms an Al—Fe—Si-based compound,during casting. A solid lubricating effect of such intermetalliccompounds prevents seizure between the material and the mold in theforming process. In the case where a content of Fe is less than 0.25%,the number of the intermetallic compounds becomes remarkably low, sothat the material sticks to a dice in a DI forming process, therebycausing deterioration of surface properties of the container. On theother hand, in the case where the content exceeds 0.6%, there arises aproblem that an excessive amount of Al—Fe—Mn-based intermetalliccompound is formed, and this compound provides a starting point ofcracking, thereby causing the deterioration of formability. The contentof Fe is preferably within a range of 0.30-0.50%.

[Si: 0.25-0.50%]

Si (silicon) forms, with the above-described Mn and/or Fe, the α phase(Al—Mn—Fe—Si-based) compound and the Al—Fe—Si-based compound, which havethe solid lubricating effect, and exhibits an effect to prevent stickingof the material to the dice in the DI forming process. The effect is notsufficiently exhibited in the case where a content of Si is less than0.25%. On the other hand, in the case where the content of Si exceeds0.50%, there arises a problem that an excessive amount ofAl—Mn—Fe—Si-based intermetallic compound is formed, so that the compoundprovides a starting point of cracking, thereby causing the deteriorationof formability, and an amount of solid-solubilized Mn is reduced,resulting in deterioration of the thermal softening resistance. Thecontent of Si is preferably within a range of 0.30-0.40%.

[Cu: 0.10-0.30%]

Cu (copper) has an effect of formation and precipitation ofAl—Cu—Mg-based intermetallic compound in the coating and baking process,so as to inhibit or prevent deterioration of the strength in the coatingand baking process. The effect cannot be sufficiently achieved in thecase where a content of Cu is less than 0.10%. On the other hand, in thecase where the content exceeds 0.30%, there arises a problem that workhardenability in the forming process becomes too high, thereby causingthe deterioration of formability. The content of Cu is preferably withina range of 0.15-0.25%.

[Zn: Not More Than 0.25%]

Zn (zinc) is an element which improves formability. However, in the casewhere a content of Zn is excessive, the cost is increased, and coarseintermetallic compounds are formed, with a result of deterioration offormability. Thus, the content of Zn is controlled to be not more than0.25%. The content of Zn is preferably within a range of 0.05-0.20%.

[Ti: Not More Than 0.10% and B: Not More Than 0.05%]

Ti (titanium) and B (boron) function to fine a casting structure so asto uniformize a dispersed state and a crystal grain structure of acrystallized product generated during casting. However, in the casewhere a content of Ti exceeds 0.10% or a content of B exceeds 0.05%,coarse intermetallic compounds are formed with a result of deteriorationof formability. The content of Ti and B are preferably within ranges ofnot more than 0.03% and not more than 0.04% respectively.

[Al and Inevitable Impurities: the Balance]

The Al alloy to form the Al alloy sheet for the can body according tothe invention consists of, in addition to the above-described alloycomponents, the balance of Al (aluminum) and inevitable impurities, thatis, elements other than the above-described alloy components. It ispreferable to minimize the amount of the inevitable impurities forreducing deterioration of properties of the sheet. The amount isgenerally set to be not more than the upper limit of each element in theAl alloy, which is defined by the JIS standard and the like. The totalamount of elements of the inevitable impurities is generally not morethan 0.15%, preferably not more than 0.10%.

The Al alloy sheet according to the invention formed of the Al alloyconsisting of the above-described alloy composition includes, after thehot rolling (before the cold rolling), not less than 0.25% by mass ofsolid-solubilized Mn, not less than 0.02% by mass of solid-solubilizedFe and not less than 0.07% by mass of solid-solubilized Si, and has anelectric conductivity of 30.0-40.0% IACS, so that the sheet exhibits ahigh degree of thermal softening resistance. A large amount of thesolid-solubilized elements results in forming of minute particles ofcompounds comprising Mn, Fe and Si during cold rolling, as describedlater. The amount of the solid-solubilized elements in the hot-rolledsheet is restricted by the content of each element in the Al alloy. Thehighest electric conductivity of the sheet achieved by the amounts ofthe solid-solubilized elements is 40.0% IACS, and even in the case wherethe additive elements are solid-solubilized by a maximum degree, theelectric conductivity of the sheet is not lower than 30.0% IACS.Meanwhile, in the case where the amounts of the solid-solubilizedelements are less than the lower limit defined above, the strength ofthe sheet is significantly deteriorated in the coating and bakingprocess.

The Al alloy sheet according to the invention initially has a tensilestrength (TS) of 280-320 MPa in a rolling direction. The tensilestrength (TS) less than 280 MPa results in a problem that the strengthof the can body formed of the Al alloy sheet is not sufficient, whilethe strength more than 320 MPa causes difficulty in the DI forming.Furthermore, the Al alloy sheet according to the invention ischaracterized in that its tensile strength (ABTS) in the rollingdirection after heat treatment at 205° C. for 10 minutes is 270-310 MPa.The tensile strength (ABTS) in the rolling direction after heattreatment less than 270 MPa results in the problem that the strength ofthe can body formed of the Al alloy sheet is not sufficient, while thestrength more than 310 MPa causes difficulty in the DI forming. Inaddition, in the Al alloy sheet according to the invention, a difference(TS-ABYS) between the tensile strength (TS) in the rolling direction anda yield strength (ABYS) in the rolling direction after heat treatment at205° C. for 10 minutes is controlled not to exceed 50 MPa, so that it ispossible to simultaneously achieve both of a desired formability of theAl alloy sheet and strength after heat treatment of the can bodyobtained by the Al alloy sheet. strength of the can body obtained by theAl alloy sheet, even after the can body is subjected to heat treatment.

To produce the Al alloy sheet according to the invention, a materialconsisting of the above-described Al alloy composition is melted to makea molten metal of the Al alloy. Then, the molten metal is formed into anAl alloy ingot, such as billet and slab, by conventional casting methodssuch as the DC casting method. It is noted that the Al alloy ingot has acomposition comprising Mn, Mg, Fe, Si, Cu, Zn, Ti and B in the amountsdefined according to the invention.

Subsequently, the Al alloy ingot is subjected to the conventionalsurface-machining and successive specific homogenization treatment so asto obtain desired properties of the Al alloy sheet according to theinvention. In such homogenization treatment, the surface-machined Alalloy ingot is heated to a homogenization temperature (T: ° C.) within arange of 550-620° C. at a heating rate of 30-120° C./h, and kept at thehomogenization temperature (T) for a time not shorter than (145-0.24T)hours (Hr). In the case where the heating rate is lower than 30° C./h,production equipment is occupied for a longer time, so that theproduction cost is increased. On the other hand, in the case where theheating rate is higher than 120° C./h, a large amount of fine particlesis formed, thereby causing problems that the strength becomes too highand the formability is deteriorated. In the case where thehomogenization temperature (T) is lower than 550° C., the effect of thehomogenization is not sufficiently exhibited. On the other hand, in thecase where the homogenization temperature (T) is higher than 620° C.,there arises a problem that the material is partly melted so that theformability is deteriorated. The homogenization treatment hascharacteristics to precipitate coarse α phase compounds so that seizurein the forming process is prevented, as well as to eliminate segregationof the elements included in the Al alloy ingot so that a uniformmicrostructure is obtained. Furthermore, the homogenization treatment iscontinued for a time not shorter than (145-0.24T) hours, whereby a crosssectional structure of the homogenized Al alloy ingot has an area of notless than 3.5% in terms of particles with a diameter of 0.1 μm-1 μm, ina microphotograph with magnification of 100-20000 times using a scanningelectron microscope. In that state, an effect of preventing seizure canbe obtained, and required amounts of solid-solubilized Mn, Fe and Si aresecured for achieving an effective thermal softening resistance. Theupper limit of the period of time of homogenization treatment isgenerally not longer than 30 hours, preferably not longer than 20 hours,in view of productivity and the like.

After the above-described homogenization treatment is finished, the Alalloy ingot can be directly (immediately) subjected to a hot rollingprocess. Alternatively, the ingot may be cooled to a startingtemperature of hot rolling not lower than 500° C. at a cooling rate of10-90° C./h, and then subjected to the hot rolling process. In the casewhere the Al alloy ingot is subjected to the cooling process such thatthe cooling rate is less (lower) than 10° C./h or the ingot is cooled toa temperature lower than 500° C., Mn, Fe and Si are precipitated in thecooling process so that the amounts of solid-solubilized elements arereduced. This causes a problem that precipitation of fine particlescomprising Mn, Fe and Si is not sufficient in a subsequent cold rollingprocess, resulting in the deterioration of the thermal softeningresistance of the Al alloy sheet. The cooling rate more (higher) than90° C./h causes a problem that a temperature distribution within the Alalloy ingot becomes uneven so that properties of an end product areunstable.

In the invention, hot rolling of the Al alloy ingot is performed as acombination of rough hot rolling and finish hot rolling, as known,whereby a sheet whose thickness after the finish hot rolling is 1.5-4.0mm is formed. The rough hot rolling is performed on the Al alloy ingotimmediately after the above-described homogenization or after thecooling to the predetermined temperature, such that a temperature upontermination of the rough hot rolling (temperature of the ingot at thetime when the rough hot rolling is terminated) is within a range of430-550° C., so as to form a sheet having a thickness of 20-40 mm. Inthe case where the above-described temperature upon termination of therough hot rolling is lower than 430° C., there arises a problem that atemperature upon termination of the finish hot rolling following therough hot rolling tends to be lower than desired. On the other hand, inthe case where the temperature upon termination of the rough hot rollingis higher than 550° C., there is a risk that coarse recrystallizedgrains are generated in the hot rolling process, thereby causingdeterioration of the formability. Furthermore, in the case where thethickness of the sheet obtained by the rough hot rolling is smaller than20 mm, there is a risk that a working ratio in the subsequent finish hotrolling is not sufficient so that an effective recrystallized structurecannot be obtained after the finish hot rolling. On the other hand, thethickness larger than 40 mm causes a problem that the working ratio inthe finish hot rolling is too large so that anisotropy in the DI formingprocess is prominent.

In the finish hot rolling following the above-described rough hotrolling, the conventional rolling process is performed such that atemperature upon termination of the finish hot rolling (temperature ofthe ingot at the time when the finish hot rolling is terminated) iswithin a range of 300-390° C. so that the sheet has a thickness of1.5-4.0 mm. In this finish hot rolling process, it is important tocontrol the obtained sheet so as to have a recrystallized structure inthe cooling process after coiling of the sheet. In the case where theabove-described temperature upon termination of the finish hot rollingis lower than 300° C., there arises a problem that formation of therecrystallized structure is not sufficient so that the anisotropy isprominent and the strength of the product is excessively high. On theother hand, in the case where the temperature upon termination of thefinish hot rolling is higher than 390° C., there is a risk that therecrystallized grains become coarse, so that the DI formability isdeteriorated. Furthermore, in the case where the thickness of the sheetobtained by the finish hot rolling is smaller than 1.5 mm, there is arisk that a working ratio in the subsequent cold rolling process is notsufficiently high, resulting in reduction of the strength. On the otherhand, the thickness larger than 4.0 mm causes a problem that the workingratio in the cold rolling process becomes excessively high, so that thestrength of the sheet is too high. By the finish hot rolling, theabove-described amounts of solid-solubilized Mn, Fe and Si are secured.

The thus obtained hot-rolled sheet is further subjected to cold rollingsuch that a total working ratio of the sheet is not less than 75% and anaverage rolling rate in a steady part of a final pass in the coldrolling process is within a range of 700-1600 m per minute, so that theobtained Al alloy sheet has a thickness of 0.2-1.0 mm. The cold rollingis performed in accordance with the conventional method. Here, if thetotal working ratio in the cold working is lower than 75% or the averagerolling rate in the steady part is higher than 1600 m/m, particles ofMn-based compounds are not sufficiently precipitated, so that thethermal softening resistance may be deteriorated. On the other hand, inthe case where the average rolling rate in the steady part is lower than700 m/m, there is a problem that productivity is significantlydeteriorated. Furthermore, the thickness of the obtained Al alloy sheetsmaller than 0.2 mm causes a problem that a sufficient strength of thecan body may not be achieved, while the thickness larger than 1.0 mmresults in a problem that the weight of the sheet is too high, so thatit is not suitable for use as a can for beverages.

Furthermore, according to the invention, the desired Al alloy sheet ispreferably produced such that the difference (S1-S2) between theelectric conductivity (S1) of the sheet (hot-rolled sheet) obtained bythe above-described finish hot rolling and the electric conductivity(S2) of the sheet (cold-rolled sheet) obtained by the above-describedcold rolling is 0.2-1.6% IACS, that is, the electric conductivity of thecold-rolled sheet is within a range of 28.4% IACS-39.8% IACS. Asufficient amount of solid-solubilized Mn allows particles of compoundscomprising Mn, Fe and Si to be induced by the cold working and finelyprecipitated, so that the thermal softening resistance is improved.Meanwhile, FIG. 1 shows a result of examination of a difference ofelectric conductivity before and after a compression forming test at150° C. equivalent to the cold working with respect to each of twosamples formed of Al alloys comprising respective different amounts ofMn. The result indicates an increase of the electric conductivity of thesample formed of the Al alloy comprising Mn after the cold working, butsubstantially no change of the electric conductivity of the sampleformed of the Al alloy not comprising Mn. Thus, the result proves thatthe Mn-based compound particles were precipitated by the cold working.Basically, in the cold working, the electric conductivity decreases dueto working deformation, but the precipitation of the particles ofMn-based compound permits an increase of the electric conductivity, sothat the amount of decrease of the electric conductivity generally fallswithin the range of 0.2-1.6% IACS. In the case where the amount ofdecrease of the electric conductivity is less than 0.2% IACS, thecold-working ratio is not sufficient so that the strength of the sheetis not sufficient. On the other hand, in the case where the amount ofdecrease is more than 1.6% IACS, the amount of precipitated Mn, Fe andSi-based particles induced by the cold rolling is not sufficient, sothat the thermal softening resistance is not sufficiently improved.

The thus obtained Al alloy sheet according to the invention is subjectedto the conventional working as necessary so as to be formed into thedesired can body, and advantageously used as an Al-based can forbeverages and the like. For example, the Al alloy sheet according to theinvention is subjected to degreasing and washing, and oil-coating, asnecessary, and further subjected to cupping, DI forming, trimming,cleaning, drying, coating, baking, necking, flanging and the like so asto obtain a can body (cylindrical can) for beverages. Then, a can lid(can end) is attached to the obtained can body, so that the desiredAl-based can for beverages is advantageously produced.

EXAMPLES

To clarify the present invention more specifically, some examplesaccording to the invention will be described. It is to be understoodthat the invention is by no means limited by the details of theillustrated examples, but may be embodied with various changes,modifications and improvements which are not described herein, and whichmay occur to those skilled in the art, without departing from the spiritof the invention.

Samples made from Al alloy sheets (original sheets: cold-rolled sheet)or their intermediate products, namely hot-rolled sheets, which wereobtained in Examples and Comparative Examples described below, weremeasured or evaluated according to the following methods.

(1) Tensile Strength (TS) of an Original Sheet in a Rolling Direction

A JIS (Japanese Industrial Standard) No. 5 sample was made from each ofthe Al alloy sheets (original sheets) obtained in the Examples andComparative Examples, with respect to the rolling direction. Each of thesamples was subjected to a tensile test in accordance with JIS-Z-2241 sothat the tensile strength (TS) of the sample in the rolling directionwas measured.

(2) Tensile Strength (ABTS) and Yield Strength (ABYS) in the RollingDirection After Heat Treatment at 205° C. for 10 Minutes

The samples formed of each of the Al alloy sheets (original sheets) weresubjected to heat treatment (at 205° C. for 10 minutes) equivalent tothe coating and baking process. Subsequently, the tensile test as in theabove (1) was performed, and the tensile strength (ABTS) and the yieldstrength (ABYS) of the samples in the rolling direction after the heattreatment were measured.

(3) Measurement of the Amounts of Solid-Solubilized Si, Fe and Mn(Phenol Dissolving Method)

A small sample piece cut out from a hot-rolled sheet after finish hotrolling obtained in each of the Examples and Comparative Examples wasimmersed in a phenol solution of 170° C. so that matrix components inthe Al alloy were dissolved, and benzylalcohol was added to thesolution. The solution was kept in a liquid state and filtered through afilter having a pore diameter of 0.1 μm. Subsequently, precipitates lefton the filter were dissolved by a mixed solution of hydrochloric acidand fluoric acid, and the obtained dissolved solution was diluted so asto be subjected to ICP (Inductively Coupled Plasma) optical emissionspectroscopy, whereby the amounts of the precipitated Mn, Fe and Si werefound. The amounts of solid-solubilized Si, Fe and Mn were calculated bysubtracting the above-described amount of the precipitation from thecontents in the ingot.

(4) Electric Conductivity

A sheet after the finish hot rolling (hot-rolled sheet) and a sheetafter the cold rolling (original sheet: cold-rolled sheet) weresubjected to measurement of the electric conductivity at a wavelength of960 kHz by using an electric conductivity measuring device (SIGMATEST2.069 available from FOERSTER Japan Limited), so that an averagevalue of n=3 was calculated. In the case where the thickness of thesample was less than 1 mm, pieces of the sample (sheet) were stackedsuch that the total thickness was not less than 1 mm, and subjected tothe measurement.

(5) Evaluation of Can Formability

Each of the Al alloy sheets (original sheets) obtained in the Examplesand Comparative Examples was subjected to cupping and DI forming at anironing ratio of 66%, trimming, and the conventional coating and bakingprocess, according to the conventional can-making method, so that thecan formability of the alloy sheet was evaluated. A state of the seizureof can walls in the can-making process was also visually examined.

Example 1

First, various Al alloys: A1-A10 having alloy component compositionsindicated in the following Table 1 were smelted according to theconventional method, and Al alloy ingots were made with respect to thealloys by the semi-continuous casting method. Subsequently, each of theobtained Al alloy ingots was subjected to the conventionalsurface-machining, heated to a temperature of 600° C. at a heating rateof 40° C./h using an air furnace, and then subjected to homogenizationat the temperature of 600° C. for 10 hours. It is noted that the valueof not shorter than (145-0.24T) hours defined according to the inventionis not less than 1 hour. Thus, the above-described 10 hours ofhomogenization period satisfies the condition.

Subsequently, after the homogenization, each of the obtained Al alloyingots was directly (immediately) subjected to hot rolling. First, theconventional rough hot rolling was performed by using a reversingrolling mill such that a temperature of the Al alloy ingots on theoutlet side of the reversing rolling mill was within a range of 460-510°C., so as to form a sheet having a thickness of 28 mm. Then, theconventional finish hot rolling was performed by using a tandem rollingmill with four stands such that a temperature of the Al alloy ingots onthe outlet side of the tandem rolling mill was within a range of300-330° C., so that the sheet had a thickness of 2.2 mm. At last, coldrolling consisting of three passes was performed so that an Al alloysheet having a thickness of 0.28 mm was produced. The total workingratio in the cold rolling process was 87.3%. The average rolling rate ina steady part of the final pass in the cold rolling process was set tobe within a range of 900-1100m/m. The temperature upon termination ofthe final pass of the cold rolling was 145-155° C.

With respect to each of the sheets formed of the Al alloys A1-A10obtained in the above-described processes, a test material (A1-A10) wasproduced, and its properties were evaluated according to theabove-described methods, the results of which are indicated in thefollowing Table 2. In Table 2, the difference (S1-S2) between theelectric conductivity (S1) of the hot-rolled sheet and the electricconductivity (S2) of the cold-rolled sheet is indicated as an amount ofdecrease of electric conductivity by cold rolling.

TABLE 1 Al Alloy components (mass %) alloy Mn Mg Fe Si Cu Zn Ti B A 10.7 1.1 0.45 0.33 0.24 0.12 0.05 0.00 2 1.3 1.3 0.38 0.32 0.24 0.17 0.000.04 3 1.2 0.8 0.50 0.43 0.19 0.19 0.05 0.04 4 0.8 1.5 0.49 0.43 0.200.16 0.01 0.01 5 0.8 1.1 0.26 0.39 0.18 0.01 0.08 0.04 6 0.9 1.0 0.590.39 0.22 0.17 0.10 0.04 7 1.2 1.1 0.33 0.27 0.21 0.10 0.04 0.03 8 1.21.1 0.38 0.48 0.24 0.04 0.07 0.01 9 1.0 1.3 0.32 0.41 0.13 0.25 0.040.00 10 0.8 0.9 0.52 0.28 0.27 0.19 0.01 0.03

TABLE 2 Sheet properties Amount of Hot-rolled sheet decrease of Solid-Solid- Solid- electric Cold-rolled sheet solubilized solubilizedsolubilized Electric conductivity TS- Test Mn Fe Si conductivity by coldrolling TS ABTS ABYS material (wt %) (wt %) (wt %) (% IACS) (% IACS)(MPa) (MPa) (MPa) Remarks A 1 0.29 0.09 0.23 39.3 1.3 288 272 44Excellent can formability 2 0.46 0.06 0.18 37.5 0.8 311 295 49 Excellentcan formability 3 0.42 0.11 0.24 39.1 0.9 305 288 47 Excellent canformability 4 0.36 0.12 0.16 38.9 1.0 310 294 48 Excellent canformability 5 0.34 0.11 0.15 38.3 1.0 291 274 44 Excellent canformability 6 0.39 0.12 0.19 39.2 1.1 289 273 47 Excellent canformability 7 0.37 0.10 0.13 37.3 1.3 306 290 44 Excellent canformability 8 0.41 0.14 0.22 37.4 0.8 303 287 40 Excellent canformability A 9 0.33 0.08 0.18 38.0 1.2 310 295 47 Excellent canformability 10 0.28 0.13 0.10 39.5 1.3 299 283 49 Excellent canformability

As is apparent from the results shown in Tables 1 and 2, the sheetsformed of the Al alloy A1-A10 had a not excessively high tensilestrength (TS) in the rolling direction in a state before the coating andbaking process, and were excellent in the can formability. In addition,the Al alloy sheets A1-A10 (sample sheets) had a high tensile strengthin the rolling direction after heat treatment (ABTS), and were excellentalso in the thermal softening resistance.

Comparative Example 1

With respect to various compositions of alloy components shown in thefollowing Table 3, under the same conditions as in the above-describedExample 1, sheets formed of each of the Al alloy B1-B13 were produced.The test material (B1-B13) obtained in the production process of the Alalloy sheets were evaluated with respect to their properties as in theabove-described Example 1. The result is shown in the following Table 4.

TABLE 3 Al Alloy components (mass %) alloy Mn Mg Fe Si Cu Zn Ti B B 10.4 1.0 0.40 0.32 0.18 0.06 0.03 <0.01 2 1.6 0.9 0.45 0.33 0.20 0.060.03 <0.01 3 1.0 0.6 0.30 0.38 0.21 0.06 0.02 <0.01 4 0.9 1.7 0.31 0.300.18 0.08 0.03 0.01 5 1.1 1.0 0.12 0.29 0.21 0.10 0.02 <0.01 6 1.0 0.90.7 0.32 0.20 0.04 0.02 <0.01 7 0.9 0.9 0.44 0.19 0.21 0.06 0.02 0.01 81.2 1.0 0.42 0.55 0.20 0.05 0.03 <0.01 9 0.9 0.9 0.44 0.27 0.02 0.060.03 <0.01 10 1.1 1.0 0.32 0.38 0.50 0.06 0.02 <0.01 11 1.0 1.2 0.330.33 0.20 0.80 0.03 <0.01 12 1.0 1.0 0.32 0.26 0.23 0.07 0.21 <0.01 131.1 1.1 0.45 0.34 0.20 0.07 0.03 0.10

TABLE 4 Sheet properties Amount of Hot-rolled sheet decrease of Solid-Solid- Solid- electric Cold-rolled sheet solubilized solubilizedsolubilized Electric conductivity TS- Test Mn Fe Si conductivity by coldrolling TS ABTS ABYS material (wt %) (wt %) (wt %) (% IACS) (% IACS)(MPa) (MPa) (MPa) Remarks B 1 0.22 0.04 0.09 41.3 1.8 283 268 54Strength of can body not enough 2 0.48 0.05 0.11 35.0 0.4 330 304 48Broken during can-making 3 0.38 0.10 0.09 38.4 1.0 276 260 49 Strengthof can body not enough 4 0.33 0.05 0.15 36.2 0.6 336 309 47 Brokenduring can-making 5 0.38 0.01 0.17 39.0 1.7 298 283 51 Roughness of cansurface 6 0.34 0.09 0.18 38.5 1.1 303 287 48 Broken during can-making 70.40 0.18 0.05 39.1 1.7 293 268 52 Strength of can body not enough 80.25 0.08 0.37 38.2 1.1 311 295 45 Broken during can-making B 9 0.390.17 0.13 38.6 1.3 291 266 53 Strength of can body not enough 10 0.400.06 0.11 38.3 1.4 328 296 42 Broken during can-making 11 0.35 0.07 0.2338.8 1.1 308 292 45 Broken during can-making 12 0.34 0.11 0.11 39.0 0.8299 283 43 Broken during can-making 13 0.37 0.16 0.23 38.8 0.9 307 29046 Broken during can-making

As is apparent from the results shown in Tables 3 and 4, the testmaterial B1, which was not subjected to a sufficient amount of additionof Mn, contained the solid-solubilized Mn less than required, so thatthe amount of precipitation of Mn in the cold rolling process was notenough and the electric conductivity of the test material greatlydecreased. As a result, the amount of decrease of strength in thecoating and baking process was significant and the strength of theobtained can was not sufficient. With respect to the test material B2,the amount of addition of Mn was excessive and the strength of theoriginal sheet was too high, resulting in a problem of breakage duringthe can-making. Furthermore, with respect to the test material B3, theamount of addition of Mn was not enough and the strengths of theoriginal sheet and the sheet after the coating and baking process werenot enough. With respect to the test material B4, the amount of additionof Mn was excessive and the strength of the original sheet was too high,resulting in a problem of breakage during the can-making. With respectto the test material B5, the amount of addition of Fe was not enough, sothat the coarse intermetallic compounds were not sufficiently formed,and the formed can had a rough surface. In addition, the amount of thesolid-solubilized Fe was not enough and the amount of precipitation ofFe in the cold rolling process was not enough, so that the strength ofthe sheet greatly decreased in the coating and baking process.

With respect to the test material B6, the amount of addition of Fe wasexcessive, so that the coarse intermetallic compounds were formed,resulting in a problem of breakage during the can-making. With respectto the test material B7, the amount of addition of Si was not enough,and the amount of precipitation of Si in the cold rolling process wasnot enough, so that the amount of decrease of strength in the coatingand baking process was significant and the strength of the obtained canwas not sufficient. With respect to the test material B8, the amount ofaddition of Si was excessive, so that the coarse intermetallic compoundswere excessively formed, resulting in a problem of breakage during thecan-making. With respect to the test material B9, the amount of additionof Cu was not enough, so that the strength was not sufficientlyincreased by the precipitation in the coating and baking process,resulting in deterioration of the strength of the sheet due to thecoating and baking process, and shortage of the strength of the canbody. Furthermore, with respect to the test material B10, the amount ofaddition of Cu was excessive and the strength of the original sheet wastoo high, resulting in a problem of breakage during the can-making. Inaddition, with respect to the test materials B11, B12 and B13, theamounts of addition of Zi, Ti or B were excessive, so that the coarseintermetallic compounds were excessively formed, resulting in a problemof breakage during the can-making.

Example 2

According to the composition of the alloy component which provides thetest material A9 (Al alloy: A9) in the Example 1, Al alloy sheets C1-C13were produced under the various conditions of production shown in thefollowing Table 5. The basic conditions of production not shown in Table5 were the same as in the Example 1. The test material (C1-C13) obtainedin the production process of the Al alloy sheets were evaluated withrespect to their properties as in the above-described Example 1. Theresult is shown in the following Table 6.

TABLE 5 Conditions of production Cold rolling Heat history beforestarting hot rolling Average Homogenization Starting Total rollingHeating Temperature Cooling temperature working rate of Al alloy rate TTime rate of hot rolling ratio final pass sheet (° C. /h) (° C.) (h) (°C./h) (° C.) (%) (m/min) C 1 35 600 5 Immediate starting of hot rolling80 850 2 110 570 10 Immediate starting of hot rolling 82 1000 3 50 56011 Immediate starting of hot rolling 85 1100 4 50 615 3 Immediatestarting of hot rolling 85 1050 5 50 570 9 Immediate starting of hotrolling 84 1300 6 60 580 23 Immediate starting of hot rolling 86 850 750 570 10 Immediate starting of hot rolling 85 1100 8 60 610 3 15 580 82850 9 50 560 11 84 518 84 900 10 50 600 5 45 510 87 1050 11 60 600 5Immediate starting of hot rolling 77 1000 12 70 580 10 Immediatestarting of hot rolling 81 800 13 40 590 10 Immediate starting of hotrolling 86 1500

TABLE 6 Sheet properties Amount of Hot-rolled sheet decrease of Solid-Solid- Solid- electric Cold-rolled sheet solubilized solubilizedsolubilized Electric conductivity TS- Test Mn Fe Si conductivity by coldrolling TS ABTS ABYS material (wt %) (wt %) (wt %) (% IACS) (% IACS)(MPa) (MPa) (MPa) Remarks C 1 0.36 0.06 0.14 39.0 1.1 287 271 46Excellent can formability 2 0.39 0.07 0.13 38.0 1.0 288 272 43 Excellentcan formability 3 0.44 0.10 0.19 37.3 0.9 296 280 48 Excellent canformability 4 0.31 0.05 0.08 38.7 1.1 299 283 46 Excellent canformability 5 0.27 0.10 0.15 37.1 1.5 295 279 42 Excellent canformability 6 0.36 0.11 0.12 37.9 0.8 302 286 44 Excellent canformability 7 0.40 0.11 0.20 39.1 1.2 319 302 43 Excellent canformability 8 0.41 0.08 0.29 37.8 0.7 288 272 44 Excellent canformability 9 0.46 0.04 0.11 39.0 0.7 301 286 43 Excellent canformability 10 0.41 0.04 0.28 37.8 0.7 311 294 40 Excellent canformability 11 0.38 0.07 0.26 38.6 0.9 297 282 44 Excellent canformability 12 0.38 0.07 0.19 37.6 0.6 314 299 45 Excellent canformability 13 0.33 0.08 0.09 37.9 1.0 289 273 49 Excellent canformability

As is apparent from the results shown in Tables 5 and 6, each of the Alalloy sheets (test materials) C1-C13 had an appropriate, not excessivelyhigh tensile strength in the rolling direction (TS) before it wassubjected to the coating and baking process, so that the can formabilityof the sheet was excellent.

Comparative Example 2

According to the composition of the alloy component which provides thetest material A9 (Al alloy: A9) in the Example 1, Al alloy sheets D1-D9were produced under the various conditions of production shown in thefollowing Table 7. The basic conditions of production not shown in Table7 were the same as in the Example 1. The test material (D1-D9) formed ofthe Al alloy sheets (cold-rolled sheets) D1-D9 were evaluated withrespect to their properties. The result is shown in the following Table8.

TABLE 7 Conditions of production Cold rolling Heat history beforestarting hot rolling Average Homogenization Starting Total rollingHeating Temperature Cooling temperature working rate of Al alloy rate TTime rate of hot rolling ratio final pass sheet (° C. /h) (° C.) (h) (°C./h) (° C.) (%) (m/min) D 1 150 600 3 Immediate starting of hot rolling84 1200 2 40 520 21 Immediate starting of hot rolling 85 1100 3 50 635 3Immediate starting of hot rolling 86 1050 4 60 560 3 Immediate startingof hot rolling 83 1000 5 60 560 12 5 530 85 1000 6 50 590 10 120 530 84950 7 60 610 3 45 480 79 950 8 50 590 10 Immediate starting of hotrolling 68 1100 9 50 610 3 Immediate starting of hot rolling 86 1800

TABLE 8 Sheet properties (cold-rolled sheet) Test TS ABTS TS-ABYSmaterial (MPa) (MPa) (MPa) Remarks D 1 325 310 47 Broken duringcan-making 2 300 285 51 Broken during can-making 3 293 277 52 Brokenduring can-making 4 311 295 51 Roughness of can surface 5 286 269 55Strength of can body not enough 6 310 294 51 Broken during can-making 7295 265 53 Strength of can body not enough 8 278 265 45 Strength of canbody not enough 9 293 265 56 Strength of can body not enough

As is apparent from the results shown in Tables 7 and 8, the testmaterial D1 had a high heating rate in the homogenization, so that fineMn, Fe and Si-based particles were formed during a rise of thetemperature, with a result of an increase of the strength of the sheet,thereby causing a problem of breakage during the can-making. Withrespect to the test material D2, the temperature kept during thehomogenization was too low, so that the homogenization effect was notenough and the structure of the sheet was ununiform, resulting in aproblem of breakage during the can-making. On the other hand, withrespect to the test material D3 whose temperature kept during thehomogenization was too high, the structure of the sheet was subjected toeutectic melting and became ununiform, resulting in a problem ofbreakage during the can-making. Furthermore, with respect to the testmaterial D4, the period of time of homogenization was shorter than(145-0.24T), so that Mn—Fe—Si-based compound particles having anequivalent circle diameter of 0.1-1.0 μm, which shows a solidlubricating effect, were not sufficiently formed, resulting in theseizure on the surface of the formed can.

With respect to the test material D5, the rate of cooling to thestarting temperature of hot rolling was low, so that the precipitationwas promoted during the cooling, the amounts of solid-solubilized Mn, Feand Si were decreased, and the precipitation of Mn—Fe—Si-based fineparticles during the cold rolling was not enough, resulting indeterioration of the thermal softening resistance of the sheet. On theother hand, with respect to the test material D6, whose rate of coolingto the starting temperature of hot rolling was high, the temperatureinside the ingot became ununiform and its material structure had avariation, resulting in a problem of breakage during the can-making. Inaddition, with respect to the test material D7, the rate of cooling tothe starting temperature of hot rolling was low, so that theprecipitation was promoted during the cooling to the startingtemperature, the amounts of solid-solubilized Mn, Fe and Si weredecreased, and the precipitation of Mn—Fe—Si-based fine particles duringthe cold rolling was not enough, resulting in deterioration of thethermal softening resistance of the sheet. As a result, the strength ofthe formed can after the coating and baking process was not sufficient.

Furthermore, the test material D8 did not have a sufficiently high totalworking ratio in the cold rolling process, so that it had an inherentproblem of insufficient strength of the can body. In addition, withrespect to the test material D9, the rolling rate in the final pass ofthe cold rolling process was too fast, so that the precipitation ofMn—Fe—Si-based fine particles during the cold rolling was not enough,resulting in a problem that the thermal softening resistance of thesheet was deteriorated.

1. A process for producing an aluminum alloy sheet for a can body, theprocess comprising the steps of: surface-machining an aluminum alloyingot consisting of 0.7-1.3% by mass of Mn, 0.8-1.5% by mass of Mg,0.25-0.6% by mass of Fe, 0.25-0.50% by mass of Si, 0.10-0.30% by mass ofCu, not more than 0.25% by mass of Zn, not more than 0.10% by mass of Tiand not more than 0.05% by mass of B, and the balance being Al andinevitable impurities; heating the ingot to a homogenization temperature(T) within a range of 550-620° C. at a heating rate of 30-120° C. perhour; homogenizing the ingot by holding the ingot at the homogenizationtemperature (T) for a time not shorter than (145-0.24T) hours; rough hotrolling the ingot immediately, or after cooling the ingot to a hotrolling starting temperature at least at 500° C. at a cooling rate of10-90° C. per hour, after finishing the homogenization, so that upontermination of the rough hot rolling, the ingot has a temperature withina range of 430-550° C., and defines a sheet having a thickness of 20-40mm; performing finish hot rolling on the sheet so that, upon terminationof the finish hot rolling, the sheet has a temperature within a range of300-390° C., and the sheet has a thickness of 1.5-4.0 mm; and performingcold rolling on the sheet so that a total working ratio of the sheet isnot less than 75%, an average rolling rate in a steady part of a finalpass is within a range of 700-1600 m per minute, and the sheet has athickness of 0.2-1.0 mm.
 2. The process for producing an aluminum alloysheet for a can body according to claim 1, wherein a difference (S1-S2)between an electric conductivity (S1) of the sheet obtained by thefinish hot rolling and an electric conductivity (S2) of the sheetobtained by the cold rolling is 0.2-1.6% IACS.
 3. The process forproducing an aluminum alloy sheet for a can body according to claim 1,wherein an area of particles having a diameter of 0.1 μm-1 μm in theingot of the aluminum alloy subjected to the homogenization is not lessthan 3.5% in terms of a microphotograph taken by scanning electronmicroscope.